Piezoelectric materials comprising ferroelectric ceramic materials have heretofore been used in piezoelectric filters, piezoelectric transducers, ultrasonic oscillators and piezoelectric buzzers. The most typical ferroelectric ceramic materials that have been used in such applications are solid solutions of PbTiO.sub.3 --PbZrO.sub.3 series. Solid solutions of Pb(Mg.sub.1/3 Nb.sub.2/3)O.sub.3 --PbTiO.sub.3 --PbZrO.sub.3 series (Japanese Patent Publication No. 42-9716) and those further containing BaTiO.sub.3, SrTiO.sub.3 and/or CaTiO.sub.3 are also known as having improved piezoelectric characteristics.
On the other hand, the use of piezoelectric ceramic materials as an actuator has also been recently studied. In this case, it is necessary to transform electric energy to mechanical energy by the displacement of the piezoelectric ceramic material itself. Accordingly, piezoelectric ceramic materials having a large piezoelectric strain constant d are desired.
The piezoelectric strain constant d, is related to an electromechanical coupling factor k and a relative dielectric constant .epsilon., of a piezoelectric ceramic material, as follows: EQU d.varies.k.sqroot..epsilon.
and therefore, in order that the material has a large piezoelectric strain constant d, it must have a large electromechanical coupling factor k and/or a large relative dielectric constant .epsilon..
Further, in applications of piezoelectric ceramic materials such as a driving part of an actuator, for example, of an ultrasonic motor, where mechanical resonance of the material is utilized, it is desired that both the piezoelectric strain constant d and the mechanical quality factor Q.sub.m are large. When a piezoelectric ceramic material is used in an ultrasonic motor, if the mechanical quality factor Q.sub.m of the material is small, heat is generated due to high frequency driving involved, frequently leading to an undesirable reduction in spontaneous polarization of the material and changes in the piezoelectric strain constant of the material with time. Accordingly, it is essential that the mechanical quality factor Q.sub.m of a piezoelectric ceramic material be large, when the material is to be used in ultrasonic motors.
While the mechanical quality factor Q.sub.m of solid solutions of PbTiO.sub.3 --PbZrO.sub.3 series or of Pb(Mg.sub.1/3 Nb.sub.2/3)O.sub.3 --PbTiO.sub.3 --PbZrO.sub.3 series can be improved by incorporation of MnO.sub.2 thereinto, there has been a problem in that the piezoelectric strain constant d of the material is drastically decreased as the amount of the MnO.sub.2 added increases. Accordingly, base solid solutions prior to the addition of MnO.sub.2 thereto should preferably have a sufficiently large piezoelectric strain constant d.
While various attempts have heretofore been made to add various oxides to solid solutions of Pb(Mg.sub.1/3 Nb.sub.2/3)O.sub.3 --PbTiO.sub.3 --PbZrO.sub.3 series optionally containing BaTiO.sub.3, SrTiO.sub.3 and/or CaTiO.sub.3 for the purpose of increasing the electromechanical coupling factor k and/or relative dielectric constant .epsilon., of the solid solutions, thereby increasing the piezoelectric strain constant d of the materials, the attainable level of the piezoelectric strain constant d has not necessarily been satisfactory.
For example, when a solid solution of Pb(Mg.sub.1/3 Nb.sub.2/3)O.sub.3 --PbTiO.sub.3 --PbZrO.sub.3 series optionally containing BaTiO.sub.3, SrTiO.sub.3 and/or CaTiO.sub.3 is incorporated with NiO, the piezoelectric strain constant d of the material is increased as the amount of NiO added is increased. However, when the amount of NiO added exceeds a certain limit, the piezoelectric strain constant d of the material is rather decreased. This is believed because whereas Ni ions preferentially enter the B sites of the Perovskite crystals represented by ABO.sub.3, if the amount of NiO admixed with a Perovskite crystals exceeds the certain limit, ions in the A sites of the crystals becomes short and, as a consequence, a part of the NiO admixed can no longer enter the B sites of the Perovskite crystals.
It has now been found that if Perovskite crystals are incorporated with metal ions which enter the A sites of the crystals in addition to metal ions which enter the B sites of the crystal, such as Ni ions, ferroelectric ceramic materials having a larger piezoelectric strain constant d are obtained when compared with the case wherein the Perovskite crystals are incorporated with metal ions which enter only the B sites. It has also been found that if the so obtained ferroelectric ceramic solid solutions having a large piezoelectric strain constant d are further incorporated with MnO.sub.2, ferroelectric ceramic materials having large piezoelectric strain constant d and mechanical quality factor Q.sub.m are obtained.